1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images used for forming electophotographic positive images from electrostatic latent images, and a process for producing the toner.
2. Description of the Related Art
Various methods for electrophotography are known in the art as described in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 and Japanese Patent Publication No. 43-24748. The electrophotographic images are usually fixed by heat, pressure or heat and pressure, or by exposing to a solvent vapor to obtain a copied article or printed matter after forming an electric latent image on a photosensitive material by a variety of methods using a photoconductive substance, developing the latent image with a toner, and transferring the toner image on a transfer material such as paper using a direct or indirect method depending on the requirement. The toner that is not transferred and left on the photosensitve material is cleaned off by various conventional cleaning methods if necessary, repeating the steps described above thereafter.
The toner as described above is usually composed of particles comprising a binder resin and colorant as main components and contains, if necessary, a charge control agent and external additive for fixing. The particle size is usually in the range of several to 20 or 30 microns. This toner is usually produced by a so-called crushing method in which colorants such as dyes, pigments or magnetic members are mixed with a thermoplastic resin and the resin is fused to uniformly disperse the colorants in the thermoplastic resin, followed by crushing of the resin and classification of crushed particles.
Recently, an image forming device using electrophotography has been widely used simply not only as a office copy machine to copy written manuscripts but also for the purposes of full color copying providing a high quality of images and high precision output for computers. Printers are now also used for personal computers as computers have come into wide use. In accordance with this trend, lowering the fixing temperature become required for the purpose of saving electric power.
Accordingly, a higher performance of the toner has been required because the requirement to form excellent images can not be attained provided that improvements in performance such as image quality and fixing ability of the toner are not satisfied.
One measure for attaining a high quality image is to make the particle size of the toner small. Improvements in the image quality and image resolution have been actually performed by making the particle size as small as several microns. However, it is difficult to make the particle size as small as 5 to 6 microns or less by the conventional crushing method since crushed debris adheres to the crushing device due to the strong impact required to make the particles as small as possible. Moreover, only a limited degree of sharpness of the particle size distribution is attained by classification of the particles due to the presence of a cohesive force caused by finely grinding the particles. As a result, controlling the electrostatic charge of the toner is difficult, thereby causing undesired scattering and fogging of images.
Toner produced by a polymerization process has been proposed to provide reduced size toner particles or to obtain a sharp particle size distribution. For example, methods for producing particles having a sharp particle size distribution in the range of 1 to 10 .mu.m are disclosed in Japanese Patent Publication No. 6-52432 and Japanese Patent Laid-Open No. 5-93002.
Although such particles having a sharp particle size distribution are excellent in fluidity, there exists another problem in that the toner tends to coagulate or agglomerate when forming close packing or, especially, when exposed to high temperatures. Once the toner or developer has agglomerated, incomplete charging is liable to occur, which leads to poor resolution of developed images.
This leaves significant problems when the glass transition point and mean molecular weight of adhesive resins are decreased so as to make finer toner particles or to lower the fixing temperature.